1. Field of the Invention
The present invention relates to a video signal transmission device, and more particularly to a video signal transmission device for transmitting video signals and having insulation between a primary circuit and a secondary circuit.
2. Description of the Related Art
Up to now, isolators were used in instrumentation as safety measures and countermeasures for noise interference. [The isolators] provided insulation for the direct current [component] of output signals and input signals. The former object is particularly important for protecting the subject body, as well as preventing damage to the instrument itself. In other words, the object was to separate the body from the primary circuit (ground circuit), which operates using alternating current from a power supply, and to insulate the portion inserted in the body in order to protect the body from electric shocks and prevent electrical stimulation from affecting another person. Safety standards (IEC660601-1, JIS T1001, UL2601-1, etc.) stipulate that the insertion portion of the electrocardiographs and the scope portion of the electronic endoscopes be insulated from the ground circuit, particularly for medical equipment such as electrocardiographs and electronic endoscopes.
On the other hand, it is now desirable to use CCDs [charge coupled devices] having many pixels (over 800,000 pixels, for example) in electronic endoscopes. When using such CCDs with many pixels, the limited transmission speeds result in low speed readout at a frame rate of 15 Hz (usual field readout is at 60 Hz). In order to output an image based on an image signal (video signal band of about 6 MHz) read at this low speed on a normal TV monitor, it is necessary to use a quadruple scan converter (scanning speed converter), for example, to convert the signal to a normal video signal with a field rate of 60 Hz. In the case of quadruple scan conversion (scanning speed conversion), however, the video signal band is also quadrupled, resulting in a video band of 6 MHzxc3x974=24 MHz, for example.
In an electronic endoscope, for example, the video signal must be output through insulation in order to ensure the safety of the subject body as discussed above. If the endoscope is not insulated from externally connected instruments (hereafter referred to as connected instruments), an electrical shock can be transmitted to the body through the connected instruments even if the electronic endoscope is itself insulated from the ground circuit. In order to prevent this, the electronic endoscope must also be insulated from the connected instruments.
The use of a photocoupler and video transformer is a known method for transmitting video signals through insulation which achieves these objects.
However, using a CCD with a large number of pixels increases the video signal band to 24 MHz, for example, as discussed above. The actual band for the video transformer and photocoupler therefore becomes less than 10 or so MHz. Sufficient transmission bandwidth cannot be ensured when these are used and the apparatus becomes difficult to use as a device for transmitting wide band video signals because of insulating the photocoupler and so forth.
The present invention was made in view of the foregoing situation. It is an object of the present invention to provide video signal transmission device for transmitting wide band video signals (for example, 20 MHz or more) across insulation between a primary circuit and a secondary circuit.
The video signal transmission device according to the present invention is a video signal transmission device having primary and secondary circuits insulated from each other and which transmits video signals between the primary and secondary circuits. This video signal transmission device comprises: differential output means, connected to either of the primary or secondary circuits, for converting a video signal to differential signals and outputting the differential signals; two capacitors, connected between the primary and secondary circuits, for respectively transmitting the two differential signals output; clamping means for restoring the direct current component of the two differential signals output; and means for restoring the video signal, connected to the other of the primary and secondary circuits, which restores the video signal based on the two differential signals output, for which the DC components have been restored.
The video signal transmission device relating to the present invention is optimal for use as a video signal transmission device for an electronic endoscope, wherein the secondary circuit is the scope portion of an electronic endoscope with imaging means on the end thereof, and the primary circuit is the video signal input/output interface portion of the electronic endoscope.
The clamping means in the video signal transmission device relating to the present invention comprises: means for outputting clamp signals, connected to either the primary or secondary circuit, for restoring the DC component of the two differential signals output; means for transmitting clamp signals for insulating the clamp signal and transmitting the clamp signal to the other of the primary or secondary circuit; and two buffers, connected to the other of the primary or secondary circuit, to which the differential signals are input; and wherein the clamping means effects DC restoration in the buffer input portion.
The primary circuit and secondary circuit noted above must be insulated from each other, but either may be referred to as the primary circuit or secondary circuit. Usually, the primary circuit refers to the ground circuit, wherein the standard voltage of the circuit is in common with the standard voltage (a neutral point in the case of a three phase source) of the AC power source. The secondary circuit refers to that which is separated from the ground circuit.
The video signal is the signal carrying the image information and it may be an RGB signal, Y/C signal, a broadcast standard NTSC signal, or the like.
The capacitors in the video signal transmission device relating to the present invention preferably comprise a circuit substrate and a pattern formed on the circuit substrate.
In this case, it is also acceptable to form the capacitors between the patterns by forming the patterns on the same surface of the circuit substrate, or by forming patterns on both sides of the circuit substrate (on the back surface, if this is a double sided substrate, for example). Moreover, the latter is preferable in view of the strength of the insulation.
Once the video signal transmission device relating to the present invention converts a video signal to differential signals, the video signal transmission device transmits the differential signals through a capacitor junction. These capacitors can therefore be electrically insulated from each other and act as a bypass filter. This makes it possible to transmit high frequency signals for which transmission of the DC component is not possible. The influence of common mode noise is alleviated because of the conversion to differential signals. Meanwhile, because the present invention is provided with clamping means, the DC component which is removed by the capacitor junction can be restored by the clamping means. It thereby becomes possible to transmit high frequency video signals, specifically wide band video signals, through insulation. As a result, electric shocks or the like are not transmitted to the body even when the device is connected to instruments which are not insulated.
Applying the video signal transmission device, relating to the present invention, to an electronic endoscope using a CCD with many pixels ensures sufficient transmission bandwidth while making it possible to output the video signal through insulation to externally connected instruments.
Furthermore, preparing the clamp signal and outputting it via the insulation with the differential signals makes it unnecessary to prepare the clamp signal on the side receiving the differential signals. Moreover, the clamping means can be realized with a simple structure because a stabilized clamp signal can be used.
In addition, forming the capacitors with a circuit substrate and patterns formed on the circuit substrate makes it unnecessary to use discrete components. It is possible to vary pattern spacing or substrate thickness or materials, making it possible to easily allow for sufficient distance to provide insulation.